1. Field of the invention
This invention relates to a resin having excellent dispersion characteristics used for magnetic recording media, which constitutes a magnetic layer that contains magnetic powder, resulting in excellent abrasion resistance, and a method for the production of the resin.
2. Description of the prior art
Magnetic recording media used for magnetic videotapes and magnetic disks, etc., are usually obtained by coating, orienting and drying a magnetic coating on a medium made of polyester, etc. The said media must have excellent magnetic characteristics, electromagnetic transforming characteristics and be durable. The magnetic coating includes magnetic powder and a resin by which the magnetic powder adheres to the media. Accordingly, resins used for magnetic recording media must have characteristics by which excellent dispersion of magnetic powder can be achieved in a magnetic coating, resulting in a magnetic layer having a high squareness ratio, and moreover, by which a magnetic layer having excellent abrasion resistance and excellent heat resistance is formed on the substrate. Especially, in recent years, the development of resins that are excellent in dispersion characteristics has become necessary with the atomization of magnetic powder based on an increased recording density thereof in the magnetic recording media.
As a resin having these excellent characteristics, vinyl chloride--vinyl acetate--vinyl alcohol copolymers (disclosed in, e.g., Japanese Patent Publication No. 61-26132), etc., are used, and when a high level of abrasion resistance of the magnetic layer is required, a resin containing the said copolymer and isocyanate compounds is used for the production of the magnetic recording media. However, these vinyl chloride--vinyl acetate--vinyl alcohol copolymers are inferior in dispersion characteristics into the magnetic coating. Vinyl chloride copolymers to be generally used for magnetic recording media are usually produced by the precipitation polymerization method as a modified method of the solution polymerization method. According to this precipitation polymerization method, since the resulting polymer is precipitated, the removal of the solvent and the residual monomers after the polymerization is readily carried out and the use of a dispersant and/or an emulsifier is not required. However, when the vinyl chloride copolymers are produced in a stainless steel reacting apparatus, a part of the copolymers deposits and adheres to the inside of a reacting apparatus, which makes the composition thereof uneven. Moreover, the removal of the deposited scales from the reacting apparatus is troublesome.